1. Field of the Invention
The present invention relates to a method of controlling the operation of a feed drive system and a spindle of an NC machine tool and, more particularly, to a control method which effectively reduces the wear and abrasion of a drill edge during a drilling with a drill.
2. Description of the Prior Art
In recent years, NC machine tools have been designed so that a spindle and a feed drive system operate at higher speeds and at higher accelerations. It is known that the higher-speed and higher-acceleration operation extends the drill life. This is supposedly because the drill of such a high-speed and high-acceleration machine tool is subjected to a slow feed process for a shorter period of time when retracted opposite to a drilling feed direction, and a drill edge is kept in friction contact with the bottom of a drilled hole for a shorter period of time than in a conventional-speed NC machine tool. This will be explained in detail with reference to FIG. 14. FIG. 14 shows velocity fluctuations in the feed rate of the drill in the vicinity of the bottom of the hole in the high-speed and high-acceleration machine tool (solid line) and in the conventional machine tool (broken line) Provided that the drill edge is in friction contact with the bottom of the hole when the feed rate is within a range of xc2x12 m/min, the friction contact lasts for about 0.13 second in the case of the high-speed and high-acceleration machine tool and for 0.26 second in the case of the conventional machine tool, as shown in FIG. 14. In the conventional machine tool, the period during which the drill edge is kept in friction contact with the bottom of the hole is longer by 0.13 second than in the high-speed and high-acceleration machine tool, so that the drill life is short.
The operations of the spindle and the feed drive system are typically controlled by a controller as shown in FIG. 15. As shown, the controller 100 includes a machining program storing section 101, a program analyzing section 102, a command generating and distributing section 103, a feed drive controlling section 104, and a spindle controlling section 105. The machining program storing section 101 stores a machining program preliminarily created. The program analyzing section 102 analyzes the machining program stored in the machining program storing section 101 to pick out commands concerning the rotation of the spindle, and the feed rate and feed position of the feed drive system from the machining program, and then sends a command signal concerning the rotation of the spindle to the spindle drive controlling section 105 and command signals indicative of the feed rate and feed position of the feed drive system 106 to the command generating and distributing section 103.
The spindle drive controlling section 105 controls a spindle drive system 107 according to the received command signal for driving thereof. The command generating and distributing section 103 determines target feed positions at regularly spaced time points for the operation of the feed drive system 106 on the basis of the received command signals and a predetermined time constant to generate operation command signals indicative of the respective target feed positions, and then transmits the operation command signals one after another to the feed drive controlling section 104. The feed drive controlling section 104 generates a velocity command signal by multiplying a deviation of a present position signal fed back from the feed drive system 106 from a received operation command signal by a position loop gain Kp. Then, the feed drive controlling section 104 generates an electric current command signal by multiplying a deviation of a present velocity signal fed back from the feed drive system 106 from the generated velocity command signal by a velocity loop gain Kv. The feed drive controlling section 104 further generates an output by multiplying a deviation of a present drive electric current signal fed back from the feed drive system 106 from the generated electric current command signal by an electric current loop gain KI, and then transmits the output as a drive command signal to the feed drive system 106. The operation of the feed drive system 106 is controlled on the basis of the received drive command signal. Although the single feed drive system is shown in FIG. 15, machine tools such as machining centers generally have a plurality of feed drive systems 106, and the command generating and distributing section 103 and the feed drive controlling section 104 are provided for each of the plurality of feed drive systems 106.
The NC machine tool (e.g., machining center) is adapted to perform a variety of machining operations such as end milling, boring, reaming and milling. In particular, the end milling is generally employed for contouring control, so that it is important to precisely control the respective feed drive systems 106 in order to achieve the contour of great precision. Even if the respective feed drive systems 106 are simultaneously driven, parameters including the position loop gains Kp, the feed velocity loop gains Kv and the cutting feed time constants for the respective feed drive systems are set at the same levels in order to prevent reduction in the precision of the contouring control. In the aforesaid high-speed and high-acceleration machine tool, the machining operations are each performed at a spindle rotation speed of 20,000 to 30,000 mxe2x88x921 or greater and at a feed rate of 10 to 20 m/min or greater, so that the time constant is set at such a level that the feed drive systems 106 are each driven at an acceleration of lower than 0.1 G during the machining operation.
In the conventional machine tool, the parameters for the feed drive systems 106 are thus set mainly for the contouring control. Therefore, the parameter settings are not necessarily optimized for the drilling in order to reduce the stagnant time of the drill in the bottom of the drilled hole for reduction of the wear and abrasion of the drill and for extension of the drill life.
In view of the foregoing, it is an object of the present invention to provide a control method for an NC machine tool, which ensures a high precision of the contouring control, and allows for extension of the drill life, and reduction in machining time.
In accordance with the present invention to achieve the aforesaid object, the present control method for an NC machine tool comprises generating an operation command signal on the basis of a machining program and a time constant, generating a velocity command signal by multiplying a deviation of a present position signal fed back from the feed drive system from the generated operation command signal by a position loop gain, generating an electric current command signal by multiplying a deviation of a present velocity signal fed back from the feed drive system from the generated velocity command signal by a velocity loop gain, and controlling a drive motor of the feed drive system on the basis of the generated electric current command signal, wherein a machining mode prescribed in the machining program is determined and, if the machining mode is a drilling mode, a predetermined operation modification value is added to the operation command signal to generate the velocity command signal and then the electric current command signal, and the drive motor of the feed drive system is driven and controlled on the basis of the generated electric current signal, when the feed drive system is driven to be retracted opposite to a drilling feed direction.
With this arrangement, where the machining mode is the drilling mode, the predetermined operation modification value is added to the generated operation command signal and the generation of the velocity command signal is based on the resulting operation command signal when the feed drive system is driven to be retracted opposite to the drilling feed direction. Therefore, the feed drive system is retracted at a feed rate higher than a rapid feed rate to be employed in a non-drilling, so that a slow feed period or a stagnant time during which the drill stays in a drilled hole can be reduced when the feed direction is reversed. Thus, a drill edge is in friction contact with the bottom of the drilled hole for a shorter period of time. This extends the drill life and, in addition, reduces a machining time. After completion of every drilling, the added operation modification value is canceled.
In the aforesaid control method for driving and controlling the drive motor of the feed drive system, the machining mode prescribed in the machining program is determined and, if the machining mode is the drilling mode, a predetermined velocity modification value is added to the generated velocity command signal to generate the electric current command signal and the drive motor of the feed drive system is driven and controlled on the basis of the generated electric current signal, when the feed drive system is driven to be retracted opposite to the drilling feed direction.
With this arrangement, the predetermined velocity modification value is added to the generated velocity command signal and the generation of the electric current command signal is based on the resulting velocity command signal when the feed drive system is driven to be retracted opposite to the drilling feed direction. Therefore, the feed drive system is retracted at a feed rate higher than the rapid feed rate to be employed in the non-drilling as described above, so that the stagnant time of the drill can be reduced when the feed direction is reversed. Thus, the drill edge is in friction contact with the bottom of the drilled hole for a shorter period of time. This extends the drill life and, in addition, reduces the machining time. After completion of every drilling, the added velocity modification value is canceled.
In the aforesaid control method for driving and controlling the drive motor of the feed drive system, the machining mode prescribed in the machining program is determined and, if the machining mode is the drilling mode, the feed drive system is driven and controlled by employing a position loop gain having a greater value than a position loop gain to be employed in the non-drilling mode when the feed drive system is driven to be retracted opposite to the drilling feed direction.
With this arrangement, the velocity command signal is generated by employing the position loop gain which has a greater value than the position loop gain to be used in the non-drilling mode when the feed drive system is driven to be retracted opposite to the drilling feed direction. Therefore, the feed drive system is retracted at a feed rate higher than the rapid feed rate to be employed in the non-drilling as described above, so that the stagnant time of the drill can be reduced when the feed direction is reversed. Thus, the same effects as described above are provided. After completion of every drilling, the position loop gain is reset to an ordinary value.
In the aforesaid control method for driving and controlling the drive motor of the feed drive system, the machining mode prescribed in the machining program is determined and, if the machining mode is the drilling mode, the feed drive system is driven and controlled by employing a velocity loop gain having a greater value than a velocity loop gain to be employed in the non-drilling mode when the feed drive system is driven to be retracted opposite to the feed direction.
With this arrangement, the electric current command signal is generated by employing the velocity loop gain which has a greater value than the velocity loop gain to be used in the non-drilling mode when the feed drive system is driven to be retracted opposite to the drilling feed direction. Therefore, the feed drive system is retracted at a feed rate higher than the rapid feed rate to be employed in the non-drilling as described above, so that the stagnant time of the drill can be reduced when the feed direction is reversed. Thus, the same effects as described above are provided. After completion of every drilling, the velocity loop gain is reset to an ordinary value.
In the aforesaid control method for driving and controlling the drive motor of the feed drive system, the machining mode prescribed in the machining program is determined and, if the machining mode is the drilling mode, a bell-shaped time constant is employed as the time constant when the feed drive system is driven in the drilling feed direction, and a linear time constant is employed as the time constant when the feed drive system is driven in a retracting direction.
With this arrangement, the operation command signal is generated by employing the bell-shaped time constant when the feed drive system is driven in the drilling feed direction, and by employing the linear time constant when the feed drive system is driven in the retracing direction. The linear time constant allows for quicker rise of the feed rate than the bell-shaped time constant. Therefore, the traveling speed of the drill can be increased when the feed drive system is driven to be retracted opposite to the drilling feed direction. As a result, the stagnant time of the drill can be reduced when the feed direction of the drill is reversed. Thus, the same effects as described above are provided. After completion of every drilling, the time constant is reset from the linear time constant to the bell-shaped time constant.
In the aforesaid control method for driving and controlling the drive motor of the feed drive system, the machining mode prescribed in the machining program is determined and, if the machining mode is the drilling mode, the feed drive system is driven and controlled by reducing the time constant to be used when the feed drive system is driven in the drilling feed direction to a value smaller than a time constant to be used when the feed drive system is driven in the non-drilling mode. The term xe2x80x9ctime constantxe2x80x9d herein means a rise time to be elapsed until the feed drive system reaches a predetermined feed rate.
With this arrangement, the time constant having a smaller value than the time constant to be employed in the non-drilling mode is employed for the generation of the operation command signal when the feed drive system is driven in the drilling feed direction. Therefore, the feed drive system is stopped at the bottom of the drilled hole at a high deceleration rate, and then its traveling direction is reversed. Thus, the stagnant time of the drill in the bottom of the hole can be reduced, so that the drill edge is in friction contact with the bottom of the hole for a shorter period of time. This extends the drill life and reduces the machining time. After completion of every drilling, the time constant is reset to an original value.
In the aforesaid control method for driving and controlling the drive motor of the feed drive system, and in a control method for controlling the rotation speed of a spindle according to the machining program, the machining mode prescribed in the machining program is determined and, if the machining mode is the drilling mode, the rotation speed of the spindle is decelerated when the feed drive system is driven to be retracted opposite to the drilling feed direction.
With this arrangement, the spindle is rotated at a lower rotation speed when the feed drive system is driven to be retracted opposite to the drilling feed direction. Therefore, a distance during which the drill edge is kept in friction contact with the bottom of the drilled hole is reduced when the feed drive system is driven to be retracted, whereby the drill life can be extended.